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<!DOCTYPE HTML PUBLIC "html.dtd"><HTML><HEAD><TITLE>Presenting XML:Resolution of the XML Specification:EarthWeb Inc.-</TITLE><META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW"><SCRIPT><!--function displayWindow(url, width, height) { var Win = window.open(url,"displayWindow",'width=' + width +',height=' + height + ',resizable=1,scrollbars=yes');}//--></SCRIPT></HEAD><BODY BGCOLOR="#FFFFFF" VLINK="#DD0000" TEXT="#000000" LINK="#DD0000" ALINK="#FF0000"><TD WIDTH="540" VALIGN="TOP"><!-- <CENTER><TABLE><TR><TD><FORM METHOD="GET" ACTION="http://search.itknowledge.com/excite/cgi-bin/AT-foldocsearch.cgi"><INPUT NAME="search" SIZE="20" VALUE=""><BR><CENTER><INPUT NAME="searchButton" TYPE="submit" VALUE="Glossary Search"></CENTER><INPUT NAME="source" TYPE="hidden" VALUE="local" CHECKED> <INPUT NAME="bltext" TYPE="hidden" VALUE="Back to Search"><INPUT NAME="sp" TYPE="hidden" VALUE="sp"></FORM></TD><TD><IMG SRC="http://www.itknowledge.com/images/dotclear.gif" WIDTH="15" HEIGHT="1"></TD><TD><FORM METHOD="POST" ACTION="http://search.itknowledge.com/excite/cgi-bin/AT-subscriptionsearch.cgi"><INPUT NAME="search" SIZE="20" VALUE=""><BR><CENTER><INPUT NAME="searchButton" TYPE="submit" VALUE=" Book Search "></CENTER><INPUT NAME="source" TYPE="hidden" VALUE="local" CHECKED> <INPUT NAME="backlink" TYPE="hidden" VALUE="http://search.itknowledge.com:80/excite/AT-subscriptionquery.html"><INPUT NAME="bltext" TYPE="hidden" VALUE="Back to Search"><INPUT NAME="sp" TYPE="hidden" VALUE="sp"></FORM></TD></TR></TABLE></CENTER> --><!-- ISBN=1575213346 //--><!-- TITLE=Presenting XML//--><!-- AUTHOR=Richard Light//--><!-- PUBLISHER=Macmillan Computer Publishing//--><!-- IMPRINT=Sams//--><!-- CHAPTER=17 //--><!-- PAGES=0309-0330 //--><!-- UNASSIGNED1 //--><!-- UNASSIGNED2 //--><P><CENTER><A HREF="../ch16/0305-0308.html">Previous</A> | <A HREF="../ewtoc.html">Table of Contents</A> | <A HREF="0313-0315.html">Next</A></CENTER></P><A NAME="PAGENUM-309"><P>Page 309</P></A><H3><A NAME="ch17_ 1">CHAPTER 17<BR></A></H3><H2>Resolution of the <BR>XML Specification<BR> </H2><B>by Simon North</B><P>The XML specification is not yet finished, nor is itcomplete. In this chapter, I review the current status of thespecification and look at the features and facets that are eitherbeing worked on now or will need to be worked on in the near <BR>future.</P><P>Let's start by quickly tracing some of XML's inheritancefrom SGML and see how it has shaped XML's approach towhat are really problems shared by them both. Following this,you'll browse through some formal and informal activities thatare either part of or contribute to the completion of thespecification of XML. To bring things full circle, I conclude byreturning to SGML and looking at how XML isreshaping SGML's future, which in turn has a direct effect onXML's future.</P><A NAME="PAGENUM-310"><P>Page 310</P></A><TABLE BGCOLOR="#FFFF99"><TR><TD>Note:</TD></TR><TR><TD><BLOCKQUOTE>I quickly skim over some pretty complex topics in this chapter.Wherever possible, I've tried to limit the technical depth to just what isnecessary. The full (technical) details of each of the topics covered in this chapterare generally publicly available on the Internet. For a full list of the URLs,see Appendix B, "Bibliography."</BLOCKQUOTE></TD></TR></TABLE><H4><A NAME="ch17_ 2">SGML on the Web</A></H4><P>Years before XML was even thought of, there was an initiative called"SGML on the Web" whose intention was exactly that—to get SGML onto theWorld Wide Web alongside (or, for preference, instead of) HTML. Themovement never really was much of a success and, now that XML has arrived, it haseither finally succeeded, or will probably never succeed, depending on howyou look at it. (An SGML purist might well consider XML to be an SGMLsell-out, while a realist might welcome any means of getting SGML onto the Web.)</P><P>The original "SGML on the Web" initiative might have either renameditself "XML," or it might have ceased to exist; however, the factors—andthe people—involved in the development of XML and the problems that haveto be overcome have remained much the same. XML has, therefore, alsobeen able to inherit some of the earlier work on "SGML on the Web" (or at leastto benefit from the earlier thought that has been given to the problems). Oneof these inheritances is a modular approach to the development activity(adopting the standard software development approach of "divide and conquer"by splitting a big problem into a lot of little ones).</P><P>The "SGML on the Web" approach divided the problem into fourseparate areas, some of which you can see returning in XML's specification:</P><UL><LI> Syntax: This is part 1 of the XML specification.<LI> Linking: This is part 2 of the XML specification.<LI> Style: This has been distributed as an intended part 3 of theXML specification, but it has not yet been submitted to the W3C asa formal proposal.<LI> Chunking: For "SGML on the Web," this was delegated to theSGML Open Consortium.</UL><A NAME="PAGENUM-311"><P>Page 311</P></A><P>The XML syntax and linking mechanisms have already been covered inPart II of this book, as has a little of XML style. The specification of XML willnot be complete until the part on style has been finalized and a chunkingmechanism has been worked out. In the first part of this chapter, we'll first lookat XML style, then look at chunking, and finally consider some of the othercontributions to completing the XML specification.</P><H4><A NAME="ch17_ 3">XML Style</A></H4><P>In May 1997, Jon Bosak, the co-editor of the XML language specification,made a personal draft of a projected style section for the XML specificationpublicly available on the Internet. (See Appendix B for the URL.) The fact thatthis document bears the thought-provoking title "XML Part 3: Style [NOTYET]" might lead you to believe that what you are about to read is a rough draftthat has been very quickly thrown together. Not so; in fact, nothing could befurther from the truth.</P><P>The XML style specification (commonly known asXS for short) is, in fact, a minor reworking of the DSSSL-o application profile. DSSSL(pronounced dissel), which stands for Document Style Semantics and SpecificationLanguage (which you'll learn a little more about later in this chapter), is a languagefor specifying the formatting of an SGML document. (It is actually an awfullot more than this, as you'll learn later, but for now this description willsuffice.) Whereas SGML has had a rough time getting accepted, DSSSL has hadan incredibly difficult time, even though it has been around for a lot lesstime than SGML. It is no exaggeration to say that DSSSL is extremely complex.To be able to do anything serious with DSSSL, you need a fair familiaritywith not just the programming language LISP, but also with a specific dialectof LISP called Scheme. Needless to say, you could probably cram theworld's supply of Scheme programmers into one room. Add to this the fact thatthere is almost no documentation for DSSSL other than the drafts of the ISOstandard (although in June 1997 a volunteer project to create a DSSSLhandbook was started on the DSSSL Internet mailing list, which is detailed in AppendixB) and, as far as large-scale adoption is concerned, you have a recipe for disaster.</P><P>The complexity of DSSSL (compounded by the fact that it describes twolanguages and not just one—the transformation language and the stylelanguage) very quickly led to an attempt to create a "cut-down" version calledDSSSL-Lite, even before the standard was published. (Voting on the draft of theDSSSL</P><A NAME="PAGENUM-312"><P>Page 312</P></A><P>standard started in August 1994; the final standard was published inApril 1996.) The DSSSL-Lite idea, in turn, gave rise to an Internet variation ofDSSSL called DSSSL-o (short for DSSSL-online) that was published inDecember 1995 and then reissued in August 1996 to include a few corrections resultingfrom changes in the final version of the DSSSL standard.</P><P>It has been said that DSSSL is to SGML, as XS is to XML, as CSS(cascading style sheets) is to HTML. I don't intend to go any further into any ofthese topics here, other than to say that the jury is still out on the matter of XS.There are, as yet, no commercial DSSSL packages, precious few noncommercialones, and no XS packages at all. (Not one SGML package supports DSSSLyet, despite various promises years ago, and none of the XML tools have madeany mention yet of support for XS.) On the other hand, a few packagessupport CSS, and CSS capability is either already present or is being built into mostof the major Web browsers. Microsoft has committed to supporting CSS,and Netscape is both directly supporting a slightly different version of CSS inits latest browser releases and indirectly committed to supporting CSSthrough its new JASS (JavaScript Style) technology. It is an unarguable fact that XSis far more powerful than CSS could ever be (without radical change), butthe best technical solution is not always the winner. We must wait to see howXS develops and what the market decides about supporting it.</P><H4><A NAME="ch17_ 4">Chunking</A></H4><P>The Internet was still very new when people started to think aboutpublishing SGML documents on it. Even then, people realized that it was going tobe necessary to publish composite documents (documents constructed outof pieces of other documents or, as we see quite often on the modernInternet, extracted from some other source such as a database and then assembled"dynamically" as circumstances require). This implied that there had to be ameans of not just combining multiple documents into a single unit, but also ofbreaking SGML into pieces (that is, chunks orfragments).</P><P>Several initiatives have been concerned with methods for splitting upSGML documents into pieces and combining the pieces into documents again.The following sections cover the four most important methods and, to helptrace the development of the common thread that unites them, I cover them inorder of seniority, from oldest to newest.</P><P><CENTER><A HREF="../ch16/0305-0308.html">Previous</A> | <A HREF="../ewtoc.html">Table of Contents</A> | <A HREF="0313-0315.html">Next</A></CENTER></P></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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